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Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or half-angle of inclination between those two lines. [1] [2] Due to foreshortening, nearby objects show a larger parallax than farther objects, so parallax can be used to determine distances.
A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale). The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. 30.9 trillion kilometres (19.2 trillion miles).
Stellar parallax is most often measured using annual parallax, defined as the difference in position of a star as seen from Earth and Sun, i.e. the angle subtended at a star by the mean radius of Earth's orbit around the Sun. The parsec (3.26 light-years) is defined as the distance for which the annual parallax is 1 arcsecond. Annual parallax ...
Parallax is an angle subtended by two lines crossing a point. In the upper diagram, the Earth (blue-filled circle) in its orbit sweeps the parallax angle subtended on the Sun (yellow-filled circle). The lower diagram shows the equal angle swept by the Sun in a geostatic model. A similar diagram can be drawn for a star except that the angle of ...
From 1891 to 1910, it had been thought this was the star with the smallest known parallax, hence the most distant star whose distance was known. Prior to 1891, Arcturus had previously been recorded of having a parallax of 127 mas.
The parallax method is the fundamental calibration step for distance determination in astrophysics; however, the accuracy of ground-based telescope measurements of parallax angle is limited to about 0.01″, and thus to stars no more than 100 pc distant. [12] This is because the Earth's atmosphere limits the sharpness of a star's image.
A similar disparity can be used in rangefinding by a coincidence rangefinder to determine distance and/or altitude to a target. In astronomy, the disparity between different locations on the Earth can be used to determine various celestial parallax , and Earth's orbit can be used for stellar parallax .
The parsec (parallax arcsecond) is defined in terms of the astronomical unit, being the distance of an object with a parallax of 1″. The light-year is often used in popular works, but is not an approved non-SI unit and is rarely used by professional astronomers. [28]